In the following description, the term pacemaker refers to all implantable cardiac devices having cardiac pacing and sensing capabilities. Briefly, in these types of devices the sensed signals are fed to a sensing amplifier for amplification and signal conditioning.
This sensing amplifier disables its sensing ability for a brief period following a sensed or a paced event. The time during which sensing is disabled is called a blanking period. The blanking period prevents inappropriate sensing of residual energy by the pacemaker amplifier following an intrinsic event or a pacemaker output pulse. The blanking period may be applied to the same chamber where the event occurs. In dual chamber pacemakers, the blanking period also may be applied to the chamber other than the one in which the event occurs. In this case, the blanking period is called the cross-channel blanking period. There are eight possible blanking periods (See FIG. 1A) in a pacemaker: (1) atrial blanking period after an atrial sense, (2) atrial blanking period after an atrial pace, (3) atrial blanking period after a ventricular sense, (4) atrial blanking period after a ventricular pace, (5) ventricular blanking period after an atrial sense, (6) ventricular blanking period after an atrial pace (7) ventricular blanking period after a ventricular sense, and (8) ventricular blanking period after a ventricular pace. The blanking period is a function of sensing/pacing polarity; sensitivity; pacing amplitude, pulse width, lead maturation, and position of leads. In general, in prior art devices, the durations of these blanking periods was either fixed at the factory, or was one of the adjustable programmer parameters that had to be set by the physician either based on average values obtained from statistical data, or by trial and error.
It is advantageous to provide dual chamber pacemaker with an AMS (Automatic Mode Switching) function, as described for example, in U.S. Pat. No. 5,441,523, incorporated herein by reference. The AMS function switches the pacemaker from a rate-response mode, wherein pacing rate is determined from a physiological pacemaker to a backup pacing rate under certain pre-selected conditions. However, in such a pacemaker an extra long atrial blanking period reduces the sensitivity of the AMS function. In the worst case situation the A-V delay may be equal to or shorter than the atrial blanking period following an atrial event (blanking periods (1) or (2)). Since the A-V delay is followed by a ventricular event, which in turn causes the atrial blanking period to extend still further by a cross channel blanking period (3) or (4). If an intrinsic R-wave occurs before the end of the A-V delay, the atrial blanking period is also extended by blanking period (3). Therefore, fast atrial events associated with atrial tachycardia such as atrial fibrillation or atrial flutter may occur during this extra long blanking period and cannot be sensed by the pacemaker. Accordingly the atrial tachycardia is not detected and the pacemaker does not activate the AMS function to switch from a dual chamber to a ventricular non-tracking mode.
However, if the blanking periods are set to be too short, in channel or cross channel noise may be erroneously sensed as a cardiac event. For example for a short atrial blanking following a ventricular event, a farfield R wave may be sensed improperly as a new ventricular event.
Similarly, if a cross channel ventricular blanking period (5 or 6) is too short, an atrial event may be erroneously interpreted as a ventricular event and ventricular pacing maybe inhibited. If the same ventricular blanking period is too long however, a premature ventricular contraction may occur during this blanking period and a proper A-V delay would not be set up.
Thus, it is clear that the operation of a pacemaker would be vastly improved if the blanking periods can be set accurately and automatically to reflect and compensate for the electrical characteristics of a particular pacemaker system and/or the patient's tissues.